Ion sensor for long term use in complex medium

ABSTRACT

Devices and methods for measuring a target ion concentration uses an electrode pair. The pair includes a working electrode and a reference electrode. The working and reference electrodes are ion-selective electrodes (ISEs). The reference ISE can include a sodium ISE. The ISE pair interacts with body fluids where a target ion concentration changes more than sodium ion concentration over time. Some ISE membranes of a pair vary essentially only in the ionophore. An ISE pair can determine the ratio of a target ion concentration to sodium ion concentration in vivo. Periodic measurement of sodium concentration in drawn blood can be used to calibrate an ISE pair and provide target ion concentration as an output. Or, a potassium/sodium ISE pair beneficially monitors potassium concentration changes over time in heart- or kidney-failure patients. Then, manual or automatic titration of a diuretic material can be implemented to maintain a desired potassium concentration.

FIELD OF THE INVENTION

The present invention is related generally to medical devices. Morespecifically, the present invention is related to ion-selectiveelectrodes. The present invention can include an implantable electrodepair having a potassium ion-selective electrode as the working electrodeand an implanted sodium ion-selective electrode as a referenceelectrode.

BACKGROUND OF THE INVENTION

Maintaining proper electrolyte concentration in the human body is notnormally a concern in a majority of healthy people, and does nottypically receive a great amount of general attention. The healthy humanbody is able to maintain an electrolyte balance within appropriatelimits.

When, due to sudden illness, chronic illness, extreme diet, ormedication, electrolytes become unbalanced, the result can be fatal. Forexample, when blood potassium concentration drops below a lowerthreshold, cardiac arrhythmia can occur, which can lead quickly tocardiac arrest and sudden death.

Heart failure is a chronic condition, in which the heart mechanicallydeteriorates over time. Heart failure can often be initiated by anon-fatal heart attack that kills cardiac muscle tissue, resulting in aweaker heart. The weaker heart does not pump as much blood with eachstroke, leading to the heart enlarging to compensate in volume for thelack of strength.]

The reduced pumping action of the heart can cause fluid to accumulate inthe body, particularly in the extremities. In order to reduce the amountof fluid accumulation, heart failure patients are often prescribeddiuretics, which reduce the amount of water maintained in the body.Diuretics work by regulating the excretion of water from the bodythrough the kidneys. Increased water voiding reduces the amount of waterheld in the body, but can also increase the amount of potassiumexcreted, thereby decreasing the potassium concentration in thepatient's blood. As previously described, low potassium levels can provefatal.

Kidney disease patients may also be prescribed medications that alterthe electrolyte balance. Even if not taking medication, patients havingkidney problems may benefit from having electrolyte balance monitored.Examples of patients that would benefit from improved electrolytemonitoring include heart failure patients on ACE inhibitors anddiuretics, hypertension patients, diabetes patients, kidney failurepatients, and patients on dialysis.

Current patient management techniques may include visits to physiciansor lab every other day to draw blood and measure electrolytes. Somesingle use, at home tests that require drawing blood (e.g. fingersticks) also exist. Applicant is not aware of any implantable,continuous potassium sensors currently available.

Many current cardiac rhythm management (CRM) products might be improvedif biochemical sensors were incorporated, to either optimize the therapyor to measure physiologic variables for diagnostic purposes. In oneexample, related to heart failure, it has been found that the risk ofboth morbidity and death among patients with severe heart failure isreduced substantially, when aldosterone receptorantagonist-spironolactone is added to standard drug therapy (inconjunction with an ACE inhibitor and/or a loop diuretic). Higher dosesof spirinolactone is needed to treat severe heart failure conditions.However, the higher the spirinolactone dose, the higher the risk ofhyperkalimia. Thus it would be highly desirable to have an in vivo K⁺sensor to optimize spirinolactone dosing. In addition, if sensors wereavailable for both Na⁺ and K⁺ to monitor kidney function, it wouldbenefit heart failure disease management significantly, since the kidneyis the main organ regulating body fluid excretion and balance.

Ion-selective electrodes (ISES) have long been used to measure ionconcentrations. One well-known ISE is a hydrogen ISE, typically known asa pH electrode. Other ISEs are also well known and commerciallyavailable. For example, ISEs exist which are selective for sodium ions.Sodium ISEs can be used on a bench top to measure sodium concentrationin blood.

Electrochemical potentials are associated with electrochemical half-cellreactions. The half-cell reactions each have a potential relative tothat of a standard half cell. Potentiometric ion-selective electrodes donot function by measuring absolute electrical potential. Rather, the ISE(the working electrode) and another electrode (the reference electrode)are coupled as a pair, and the differential electrical potential betweenthe two is used as an indication of ion activity near the ISE.

The reference electrode used with an ISE is often a Ag/AgCl (metal/metalhalide) electrode immersed in a saturated KCl solution, at definedconcentration, pressure, and temperature. The KCl solution may in turnbe immersed in, and in fluid communication with another solution, a saltbridge. The KCl can be depleted over time, and the KCl refilled througha port in the reference electrode designed for that purpose. The size ofthe KCl reservoir can be reduced, with an attendant reduction in timebetween required refills. This reference electrode is typically quitelarge, complicated to fabricate, and prone to drift in potential causedby samples and environment.

Current reference electrodes are thus not optimally suited for long termimplantation. Applicant believes that both the reference electrode sizeand refill requirement has posed significant challenges to developimplantable ISEs, due to the need for a reference electrode, and thecurrent lack of suitable implantable reference electrodes.

What would be desirable are small ISE electrode pairs suitable forimplantation into a human body that do not require a conventionalreference electrode.

SUMMARY OF THE INVENTION

The present invention provides methods for measuring a potassium ionconcentration in a human body, where the method can include obtaining adifferential electrical potential between an implanted potassiumion-selective electrode (ISE) and an implanted sodium ISE, thendetermining a sodium ion concentration near the sodium ISE. The methodcan also include determining a potassium ion concentration at least inpart as a function of the sodium ion concentration and the differentialelectrical potential.

The method may also include determining the sodium ion concentration bymeasuring the sodium ion concentration in human body fluids. Somemethods may include determining the sodium ion concentration using afunction of at least the measured sodium ion concentration and a changein the differential electrical potential since the time of measuring thesodium ion concentration. Some methods include determining the sodiumion concentration by using an expected change based on therapy performedon the human body since the sodium measuring. The method may includedetermining the potassium ion concentration by application of a Nernsttype equation utilizing the log of the ratio of the potassium and sodiumion activities. Body fluids that can be measured include blood, urine,and spinal fluid.

The present invention also provides methods for monitoring electrolytechanges in a human body fluid, where the method can include obtaining adifferential electrical potential from an electrode pair implanted inthe human body, the electrode pair including a potassium ion-selectiveelectrode (ISE) and a sodium ISE. This method can also include obtaininga ratio of potassium ion concentration to sodium ion concentration atleast in part as a function of the differential electrical potential.

The method can further include determining a sodium ion concentrationfrom measuring sodium ion concentration in the human body fluid, anddetermining a potassium ion concentration at least in part as a functionof the determined sodium ion concentration and the differentialpotential. The method may include repeating the obtaining step over timeto monitor changes in the ratio over time. Some methods includegenerating an indication when the potassium to sodium ratio drops belowa lower threshold.

One device according to the present invention is an implantableelectrode pair including a potassium ion-selective working electrode anda sodium ion-selective reference electrode. The device may have thepotassium ion-selective electrode including a first substrate, a firstmetal/metal-halide layer disposed on the first substrate, and a firstpotassium ISE membrane disposed over the metal/metal halide layer. Thesodium ion-selective electrode can include a second substrate, a secondmetal/metal-halide layer disposed on the second substrate, and a secondsodium ISE membrane disposed over the second metal/metal halide layer. Ahalide (reference electrolyte) containing layer may be interposedbetween the metal/metal-metal halide layer and the ISE membrane. Thehalide containing layer may be a hydrogel layer containing KCl as thereference electrolyte, disposed between the metal/metal halide layer andthe ISE membrane.

In some devices, the implantable electrode pair first and secondsubstrates are the same. The first and second metal/metal halide layermay have the same chemistry in some devices. The first ISE membrane mayhave a potassium ion-selective ionophore and the second ISE membrane mayhave a sodium selective ionophore, depending on the embodiment. In somedevices, the first and second ISE membranes include a polymer in whichthe polymer is substantially the same for both ISE membranes.

In some electrode pairs, the first and second ISE membranes each includea mobile plasticizer in the polymer. In some electrode pairs, the firstand second ISE membranes are self-plasticizing, having essentially nomobile plasticizer in the polymer. In some electrode pairs the potassiumor target ion ISE and sodium ISE membranes are each contained within asurface area of less than about 1 square centimeter. In some implantableelectrode pairs, the potassium and sodium ISE membranes each have nomajor dimension greater than about 1 centimeter.

Methods for monitoring a ratio of potassium ion to sodium ionconcentration in blood in a human being over time are also included inthe present invention. The method can include measuring a differentialelectrode potential between a potassium ISE and a sodium ISE both incontact with blood, obtaining the ratio at least in part as a functionof the measured differential electrode potential.

Methods for controlling delivery of a substance into the human body arealso included in the present invention. The methods can includemeasuring a differential potential from an ion selective electrode (ISE)pair implanted within the body, where the ISE pair includes a first ISEresponsive to a first ion concentration electrically coupled to a secondISE responsive to a second ion concentration. The method can alsoinclude varying delivery of the substance at least in part as a functionof the measured differential electrical potential.

In some methods, the first ion is potassium and the second ion issodium, the first ISE is a potassium ISE, the second ISE is a sodiumISE, and the measuring includes measuring the differential electricalpotential between the potassium ISE and the sodium ISE. The method mayfurther include varying delivery by varying infusion from an implanteddelivery device operably coupled to the ISE pair. In some methods, thesubstance includes a diuretic and the varied delivery includes varyingdelivery of the diuretic. The delivery device is located external to thebody in some methods.

Implantable ion-selective electrode pairs are also within the scope ofthe invention. The ISE pairs can include a first substrate having afirst electrical conductor in ion communication with a firstion-selective layer, in which the first ion-selective layer includes afirst membrane (which may contain a first polymer) and a firstionophore, and in which the first membrane/polymer is self-plasticizing.The ISE pair can also include a second substrate having a secondelectrical conductor in ion communication with a second ion-selectivelayer, and in which the second ion-selective layer includes a secondmembrane (which may include a second polymer) and a second ionophore, inwhich the second membrane/polymer is self-plasticizing.

In some implantable ion-selective electrode pairs the first and secondsubstrates are the same or substantially the same. In some ISE pairs,the first and second ISE membranes consist essentially of the samecomposition, but have different ionophores as between each other. Thefirst and second ISE membranes may have different ionophoreconcentrations as between each other. In some ISE pairs, the first andsecond polymers are polymers with a polymer backbone and a plurality ofpendant lipophilic plasticizing groups. The polymer may have a Tg ofless than −10° C. Some lipophilic plasticizing groups include C3-C7 orC3-C20 alkyl groups.

Some sodium ISEs exhibit a substantially sub-Nernstian response over aphysiological range of sodium ion concentration, showing a less thanNerstian change in electrical potential with change in sodiumconcentration, thus providing a more constant reference electrodepotential. Some sodium ISEs have a molar ratio of ionophore tolipophilic salt of between about 1 and 1.1.

The present invention also provides a method for detecting ischemiaincluding obtaining a differential electrical potential between animplanted electrode pair in the human body, the electrode pair includinga first ISE and a sodium ISE. The method can further include generatingan indication of ischemia at least in part as a function of a change inthe differential electrical potential over time. In some methods, thefirst ISE is a pH ISE, in which the change is a drop in pH over time.Some methods utilize a potassium ISE as the first ISE, where the changeis a rise in a ratio of potassium to sodium over time. The electrodepairs can be implanted in the or near the heart or brain, to detectcardiac infarction or stroke, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a prior art, bench top system formeasuring ion activity using a working ISE and a reference electrode;

FIG. 2 is a schematic view of a prior art, thick film system formeasuring ion activity using a working ISE and a reference electrode;

FIG. 3 is a schematic view of a thick film system of the presentinvention for measuring ion activity using a working ISE and a sodiumISE reference electrode;

FIG. 4A is a schematic, perspective view of a lead having an ISEelectrode pair including a sodium ISE as the reference electrode;

FIG. 4B is a schematic, perspective view of a lead having a planar ISEelectrode pair including a sodium ISE as the reference electrode;

FIG. 5A is a detail view of an ISE pair sensor of the present inventiondisposed within a narrow body vessel having thrombus and a capsuleformation near the sensor;

FIG. 5B is a schematic, cutaway view of a heart having an ISE pair leaddisposed within and coupled to an implantable medical device (IMD); and

FIG. 6 is a schematic view of a drug delivery device controlled by anISE pair.

DESCRIPTION OF VARIOUS EMBODIMENTS

FIG. 1 illustrates a prior art, typical bench top potentiometricmeasurement setup 20, as is currently used to measure small ions such asNa⁺, K⁺ and H⁺ using a potentiometric method. A specific ion activity ismeasured using a working ion-selective electrode (ISE) 24 against areference electrode 22.

Ion-selective electrode 24 includes an internal electrode 50 having anAg/AgCl (or Hg/Hg₂Cl₂) electrode immersed in an inner filling solution52 contained within ISE vessel or chamber 54 and bounded by an ISEmembrane 56, disposed between inner filling solution 52 and samplesolution 26.

Working electrode 24 ISE membrane 56 is typically made of plasticizedpolymer membrane containing ionophore that is highly selective for thetarget ion being sensed. Different ion activity/concentration in samplesolution 26 will produce different electrical potential on the workingelectrode membrane surface, and this potential has to be measuredconventionally against reference electrode 22. The potential ofreference electrode 22 should be constant and independent from changesof the sample ion activity/concentration.

Reference electrode 22 shown in FIG. 1 is of a frequently used design.Reference electrode 22 includes a Hg/Hg₂Cl₂ electrode 28 (which may be aAg/AgCl electrode in other systems). Hg/Hg₂Cl₂ electrode 28 is immersedwithin a vessel or chamber 29 having a diaphragm 32 and containingsaturated KCl as a reference electrolyte, indicated at 30. A salt bridgevessel or chamber 36 forms a salt bridge 34 containing a salt solutionand extending through a capillary 42 into sample solution 26. A workingelectrode conductor 60 and a reference electrode conductor 58 allow thedifferential EMF to be measured as indicated at 62. The standardhalf-cell nomenclature for system 20 is given at 57.

To maintain the constant reference electrode potential, referenceelectrode 22 typically requires a constant Cl⁻ concentration in innerreservoir 29 housing Hg/Hg₂Cl₂ electrode 28 (or a Ag/AgCl electrodeinstead), and also requires a constant flow of high concentrationelectrolyte (inner filling solution) such as KCl into the samplesolution, to minimize the so-called liquid junction potential (LJP). Aminimum flow rate is required to overcome counter diffusion of anyinterference ions into reference electrode chamber 29. A driving forcelike gravity or positive pressure is needed to maintain such a positiveand constant flow. With a limited reservoir, frequent inner solutionrefilling of chamber 29 is necessary. To reduce the refilling frequency,a restriction element like a frit or porous membrane 32 is placed tolower the flow rate. However, dissolved salt from inner electrode 28 andprotein or cells from sample solution 26 can block the porous structureof diaphragm 32 and thus introduce LJP variation, or even result incomplete malfunction of reference electrode 22.

The size of the inner filling solution reservoir 29 cannot be too small,otherwise frequent refilling is still necessary even with restrictionelement 32 in place. This poses a great challenge to miniaturize thesensor system as a whole. Without the clogging or contamination of therestriction element, while under tight lab control on the bench, such areference electrode can provide a constant potential reference within a1 mV variation. This type of reference electrode is complicated indesign, demands frequent maintenance, and is hard to miniaturize.Consequently, these difficulties present obstacles for in vivoapplications, especially for chronic in vivo applications.

FIG. 2 illustrates another common prior art ISE system 70. System 70includes a reference electrode 75 and a working electrode 73 formeasuring ion activity in a sample solution 78, both based on thickfilms and both disposed on a substrate 71. Conductors 86 and 88 allowthe differential electrical potential to be measured, as indicated at90.

Working electrode 73 includes a Ag/AgCl layer 72 having a hydrogel layer74 disposed over the Ag/AgCl layer. An ISE thick film membrane 76 isdisposed over the hydrogel layer. Hydrogel layer 74 typically contains afixed amount of electrolyte, e.g., KCl.

Reference electrode 75 includes a Ag/AgCl layer 80 having hydrogel layer82 disposed over the Ag/AgCl layer. A non ion-selective porous membrane84 is disposed on top of hydrogel layer 82. This non ion-selectiveporous membrane will act as restriction membrane to limit the exchangeof ions from sample 78 into hydrogel layer 82, which would change thereference electrode potential, and to limit diffusion of the electrolyteincorporated in the hydrogel layer out of membrane 84 and into samplephase 78.

Sensor system 70 has been widely used for point of care electrolyteone-shot tests. Applicant believes that continuous use of this kind ofsensor in complex media, like blood, serum or urine, or use insidetissue will face the same challenges of very limited reference electrodestability and short limited use life, owing to free diffusion ofinterfering ions from the sample into reference electrode membrane 84.In addition, since membranes 76 and 84 are made from different materialsin most cases, the amount of protein absorption into them, and theiradhesion to underlying hydrogel, etc. will be different. This differentbehavior can cause sensor signal drift, signal variation, or error.Applicant believes that this kind of sensor will not be able to functioncontinuously for a long period of time, either in vivo or ex vivo, owingto its reference electrode limitations.

Another problem in using this type of sensor is frequent biofouling orlimited biostability. In addition to protein absorption caused potentialdrift and/or signal error, when the ion sensor is in contact with bloodor is implanted inside the body, thrombosis formation and tissueencapsulation may cause sensor unreliability and even malfunction. Thus,poor stability is a large problem, and should be significantly improvedin order for ion sensors to be used long term in complex media likeblood or urine, particularly in vivo.

Applicant believes that previous researchers and developers have beenheavily influenced by conventional teaching, that a conventionalreference electrode (e.g. AgCl or Hg₂Cl₂) is to be used for measuringion activity using ISEs.

FIG. 3 illustrates an ISE system 100 according to the present inventionthat does not require use of a conventional reference electrode. System100 will be discussed below using the example of a system for measuringK+ ion activity in a sample 108, although the present invention is notlimited to measuring the K+ ion. System 100 includes a working ISE 102and a reference ISE 104 which may both be formed on a substrate 106 insome examples of the invention. Substrates may be polymeric, ceramic, orany other suitable material. Working electrode 102 can include a Ag/AgCllayer 110 having a hydrogel layer 112 disposed over the Ag/AgCl layer.Hydrogel layer 112 can contain a reference electrolyte, for example,KCl. An ISE membrane 114 can be disposed over the hydrogel layer. ISEmembrane 114 can be selective for K+ ions.

Reference electrode 104 can include a Ag/AgCl layer 116 having ahydrogel layer 118 formed over the Ag/AgCl layer. Hydrogel layer 118 cancontain a reference electrolyte, for example, KCl. A reference ISEmembrane 120 can be formed over the hydrogel. Membrane 120 can beselective for Na+ ions. A working electrode conductor 122 can be coupledto Ag/AgCl layer 110 and a reference conductor 124 can be coupled toAg/AgCl layer 116 to produce a differential electrical potential asindicated at 126.

Operation of the present invention may now be explained. Applying thewell-known Nernst equation to the sodium (Na) and potassium (K) ionsystems, we have:ΔE _(Na) =E _(Na) ⁰+59*log(a _(Na))ΔE _(K) =E _(K) ⁰+59*log(a _(K))thereforeΔE _(K/Na) =E ^(0′)=59*log(a _(K) /a _(Na))where E^(0′)E_(K) ⁰−E_(Na) ⁰

If the Na ion activity is relatively constant over time, or the relativechange is small compared to the relative change of the K ion activity,then the K ion activity changes over time may be monitored using system100 of FIG. 3. This is because the ΔE_(K/Na) will effectively trackchanges in ak. If the Na ion activity is relatively constant over timeand the Na ion activity can be periodically measured using conventionalblood drawing and analytical techniques, then the absolute K ionactivity may be accurately monitored by calibrating the system. Trendingof K change is possible as it follows from the above equation that, formeasurements taken at a time 1 and a time 2, that:ΔE _(2/1)=59*log((a _(K2) ·a _(Na1))/(a _(K1) ·a _(Na2)))=ΔE _(k2/k1)−ΔE _(Na2/Na1)

Hence a traditional reference electrode need not be used at all, insteada second ion-selective electrode will be used as a reference electrode.The potential change of the second ion-selective electrode should besmall when using it in the intended application environment. The ratioof the ion pair, for example, Na/K, may itself be of high interest andclinical value, as in the case of kidney function monitoring. Forexample, normally sodium change in vivo will contribute to a muchsmaller potential change vs. the change of potassium, so the potassiumchange can be inferred indirectly from the K⁺/Na⁺ ratio measurement.

Blood sodium concentration normally can range from about 137 to about148 mEq/L. Taking the ratio 148/137 and applying the Nernst equation,this corresponds to a band of about 2.7 mV. Blood potassium normally canrange from between about 3.7 to 4.8 mEq/L. Taking the ratio 4.8/3.7 andapplying the Nernst equation yields a band of about 12 mV. The absolutevariability of the potassium concentration is less than that for sodium,but the relative variability is greater than that of sodium.

Therefore, even with some variation in sodium concentration, the log ofthe ratio of the concentration of potassium to sodium is stillindicative of the potassium concentration. Trends in potassiumconcentration changes over time may be tracked using the K+/Na+ ratioobtained from the K+/Na+ ISE pair. Where only extremely low or extremelyhigh K+ is important, the K+/Na+ ratio, together with an inferred,approximate, or previously measured Na+ concentration, can produce aclinically useful indication of a dangerous K⁺ level. Where the Na⁺concentration can be accurately periodically obtained from bench top orat home measurements of drawn blood, then the K+/Na+ ratio provides afairly accurate K+ concentration value.

The ISE pair values can be telemetrically transmitted from implantedmedical devices to in-home patient management systems, which may besimilar to those currently in use to monitor implanted cardiac pacingand monitoring systems over telephone lines.

The biofouling effect may also be minimized. Since the polymer matrix ofthe K⁺ ion-selective membrane and the Na⁺ ion-selective membrane can bealmost identical, except for a difference in ionophore, the extent ofprotein adsorption and cell adhesion are expected to be almost equal,and the differential measurement scheme proposed here will cancel theseeffects out to a large extent. In contrast, in FIG. 2, the referenceelectrode membrane is a porous membrane and is typically quite differentfrom the ion-selective membrane. For this reason, the biofouling processis expected to be quite different on the polymer membrane surfaces forthe reference and working electrodes in FIG. 2.

The sensor drift can also be minimized. Sensor drift can be caused bypolymer membrane deterioration, e.g., lipophilic salt leaching, neutralpeptide/surfactant absorption, water absorption/swelling and adhesion tothe substrate. This drift can be very similar for almost the identicalpolymer membranes of the two ISE membranes in FIG. 3. The two ISEmembranes shown in FIG. 3 provide a differential measurement of the ionratio (in this case K⁺/Na⁺), and can thus minimize this kind of drift,as the two membranes will likely be affected similarly. This drift hasbeen very hard to be deal with when using a traditional referenceelectrode in the system.

The similarity of the two polymer membranes can also provide designshaving easier construction. Construction of both the primaryion-selective sensor (the working electrode) and the secondaryion-selective sensor (the reference electrode) can be very similar.

FIG. 4A illustrates a lead 130 including a lead shaft 132 having asymmetric sensor pair carrying a primary ISE 138 (the working electrode)for measuring K+ ion and a secondary ISE 134 (the reference electrode)for measuring sodium. Conductors 136 and 140 can carry the differentialelectrical signal from shaft 132 to be measured as indicated at 142.Applicant believes the symmetric sensor pair will be easier tomanufacture than more dissimilar electrode pairs. The symmetricarrangement will help to reduce non-specific effects, such as drift orbiofouling.

FIG. 4B illustrates another symmetric ISE pair 150 including a firstplanar ISE 152 for measuring K+ and a second ISE 154 for measuring Na+.The pair is coupled through conductors 156 and 158 to have thedifferential electrical potential measured as indicated at 160.

FIG. 5A illustrates an ISE pair 162 according to the present inventionimplanted in a narrow artery space 164. Thrombus formation and capsuleformation is indicated at 166. In previous efforts, with traditionalreference electrode designs, when an ion-selective sensor/biosensor wasimplanted in narrow artery space, the thrombosis formation and tissueencapsulation often caused both reference electrode and workingelectrode to not work properly, or to even stop functioning completely.Without the complication of the traditional reference electrode, theeffect of thrombosis formation and tissue encapsulation is expected tobe reduced or minimized, as previously explained, using some devicesaccording to the present invention.

FIG. 5B illustrates an implanted medical device (IMD) 170, which can be,for example, a pacemaker or an implantable cardio defibrillator. IMD 170includes a housing or can 172 and two leads 174 and 175. Lead 174extends into a heart 176, through right atrium 178 and right ventricle177, to terminate in a conventional electrode 180 disposed against theright ventricle wall 179. Lead 175 also extends into heart 176 andterminated in an ISE pair 182 placed at a high blood flow site. The highblood flow site can be, for example, the right ventricular outflow tract(RVOT) or the super vena cava (SVC), to minimize thrombosis formationand tissue encapsulation. Placing a biochemical sensor at blood highflow region, like RVOT or SVC, advantageously avoids or at least reducestissue encapsulation or thrombosis formation (as shown in FIG. 5A) onthe sensor. Tissue encapsulation or thrombosis formation on the sensorsurface will deleteriously creates extra diffusion barrier(s) foranalytes to reach the sensor surface, thus delaying the sensor responseto true analyte variation in the blood stream. The metabolism ofencapsulated tissue/cells may also generate local analytesconcentration, which may be quite different from the circulating blood.This causes sensor output irrelevant to the disease progression andsensor signal becomes unreliable.

FIG. 6 illustrates a drug delivery device 200 having a housing 202 and aseal or septum 204 for filling the device with the drug to be delivered.Device 200 also includes a tube 208 having a distal port 210 fordelivering the drug and coupled to housing 202 with a strain relief 206.In this example of this aspect of the invention, an ISE pair including aNa ISE 212 and a target ISE is disposed on the drug delivery tube. Inother embodiments, the ISE pair can be disposed on a separate lead, asillustrated in FIG. 5B.

Device 200 represents a drug delivery device generally, both externalhaving an internal delivery tube, and implantable devices. Device 200can be similar in some respects to externally worn insulin deliverydevices. Device 200 may be similar in some respects to implantable drugdelivery devices, such as the Synchromed drug delivery device (availablefrom Medtronic, Minneapolis, Minn.). Device 200 can include a controllerwithin for controlling the drug delivery responsive to the differentialelectrical potential from the ISE pair. The device may control avariable orifice opening, the opening and shutting of an open-shutvalve, the delivery rate of a metering pump, and other delivery ratecontrol devices well known to those skilled in the art. The drugs orsubstances delivered can be drugs previously discussed, including, forexample, diuretics.

In another aspect of the present invention, self-plasticizing ISEmembranes may be used. Plasticizers are typically used in ISE membranesand may form a considerable portion of (e.g. even half) of the membraneweight. The plasticizer allows the analyte to migrate through the ISEmembrane. With time however, the plasticizer and the ionophore may bothleach from the membrane. This may degrade the performance of the ISEover time.

Some embodiments of the present invention include an implantable ISEpair having ISE membranes that are self-plasticizing. Some embodimentmethods include the in vivo measuring of ion ratios using a Na ISE asthe reference electrode, where both ISE membranes have at least someself-plasticizing functionality. In some ISE membranes according to thepresent invention, there is no, or essential no unbound plasticizers inthe ISE membranes.

According to this aspect of the invention, the present invention canprovide a selective electrode for use in measuring the concentration ofan ion in solution comprising an electrode polymer membrane which isselectively permeable to the substance to be measured, where themembrane is formulated from a polymer with a backbone, which may be anacrylate backbone and have a plurality of pendant lipophilicplasticizing groups.

The membrane can be formulated from a polymer having an acrylatebackbone and a plurality of pendant lipophilic plasticizing groups toprovide the polymer with a Tg of −10° C. or less. The Tg of the polymercan be measured directly using any suitable apparatus. The polymer Tglies in the range from −10° C. to −70° C., or from −30° C. to −60° C.,in some embodiments. The lipophilic plasticizing groups are C₃₋₇ alkylgroups in some polymers used in the present invention. Use of C₃₋₇ alkylacrylates in the polymer can provide a polymer that is inherently softand does not require added plasticizer, i.e. the polymer is in effectself-plasticizing, so that the problem of leaching of the plasticizerdoes not arise.

Materials such as ceramics, metal-oxides, glass, and polymers can beused in the membrane carrying the ionophore. Some polymers can includepolyurethane, PVC, and silicone rubber. The polymer used according tothe invention can have an acrylate backbone and can be a polymer orcopolymer of one or more of the following monomers: propyl acrylate,butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate. Thepolymer may be a homopolymer or may be a co-polymer including two ormore different monomer units. The different monomer units may be derivedfrom C₃₋₇ alkyl acrylates as described above or the polymer may includea minor proportion of other monomer units, for example up to 10% byweight of monomer units derived from lower (C₁ or C₂) alkyl acrylates ormethacrylates, for example methyl methacrylate. In the case of C₆ or C₇alkyl acrylates, it may be advantageous to use this monomer in admixturewith a lower, for example C₃ or C₄ alkyl acrylate. For example, amixture of 70% heptyl acrylate and 30% butyl acrylate may be used.Straight chain alkyl acrylates may be used. Branched chain alkylacrylates or alpha or beta substituted monomers tend to produce apolymer of higher Tg than polymers produced from the correspondingstraight chain or non-substituted monomer.

It may be desirable to adjust the hydrophobic/lipophobic balance (HLB)of the polymer so as to be compatible with a particular ionophore orother analyte receptor which it is desired to incorporate into thepolymer. HLB can be adjusted by including monomers containing hydroxylgroups, for example hydroxy ethyl methacrylate.

The ionophore may be incorporated into the monomer mixture used toproduce the polymer, thus trapping the ionophore in the polymer matrix.Alternatively the ionophore may be secured chemically by grafting to thepolymer backbone. In general, the more hydrophilic the polymer, the morethe ionophore will be subject to leaching in which case it is especiallyadvantageous to graft the ionophore to the polymer backbone.

Good ISEs generally try to exhibit Nernstian response to the target ionconcentration. In the present invention, for the sodium ISE, it may bedesirable to have sub-Nernstian response. In particular, it may bedesirable to have the sodium ISE exhibit a flater slope with respect tochange in sodium concentration than normal, over the physiological rangeof sodium concentration, such that the body fluid sodium ionconcentration changes do not follow the 59 mV slope, but rather a flaterslope than normal. This would provide a more constant potential from thesodium ISE acting as the reference electrode.

Applicant believes that the sodium ISE may be made to exhibitsub-Nernstian response by making the polymeric membrane more hydrophilicthan normal. This may be accomplished by the incorporation of hydroxylgroups into the polymer, as one example. Polymer chemists skilled in theart will know how to modify existing polymers to be more hydrophilic.Applicant also believes that the sodium ISE polymer may be adjusted tooperate closer to, but under, the point of the Donnan exclusion failure.Some sodium ISEs have a molar ratio of ionophore to lipophilic salt ofbetween about 1 and 1.1.

Some ISE membranes according to the present invention can include apolymer matrix, and active components such as ionophores, salt additivesor enzymes that can enhance sensor stability and use life. Conventionalsensor materials and components may include polyvinyl chloride,polyurethane, silicone rubber, and various kinds of plasticizers,lipophilic salt additives and ionophores incorporated within thesepolymers.

The ISE pairs can measure ion pairs of interest, for example, pH/Na+,Cl−/Na+, K+/Na+, etc. in vivo/ex vivo, either acutely or chronically.Some embodiments of the present invention include implanting multipleion sensors at multiple sites, by comparing ion activity or ion pairratio (e.g. K⁺/Na⁺), at the same time or different times (e.g., trendinganalysis), to analyze the function of the same organ at different parts:e.g., within the heart, at RVOT versus deep inside coronary sinus; or ofdifferent organs, e.g., heart versus kidney; or to analyze differentmedia, e.g., venous blood versus arterial blood, or venous blood versusurine (for kidney function analysis), or to monitor tissue/organ diseaseprogress, e.g., inside versus outside cancer tumor, or to put the sensorinto artificial organs (e.g. heart, kidney or liver) to monitor theseartificial organs' performance in vivo or ex vivo. The sensor may beused if the ratio of electrolyte pair is itself of interest or clinicalimportance. The sensor can also be used in the blood, in the kidneydialysis loop, in extracorporeal life support systems (e.g. heart-lungmachines, dialysis machines), or even in instruments monitoring cellgrowth and death.

The present invention may be used in drug dosing: e.g., foradministering spironolactone, ACE inhibitor and diuretics. Some examplesof the invention may be used to provide warning ofhyperkalimia/hypokalimia. Some devices and methods according to thepresent invention may be used to monitor organ status: e.g., heartinfarction, remodeling, kidney failure, or hypertension. Sodium ISEsaccording to the present invention may be used as the referenceelectrode in various in vivo enzymatic biosensors to measure, forexample, glucose, urea and creatinine.

The present invention also provides a method for detecting ischemia.When ischemia occurs, the drop in oxygen and rise in carbon dioxideleads to a rapid drop in pH. Applicant believes this drop in pH may beobserved in both the arterial blood downstream of the blockage andsomewhat in venous blood fed through arterial then venous capillariesnear the blockage. After cell death has occurred, potassium may bereleased from the affected cells into the blood stream, increasing thepotassium concentration the blood. The drop in pH, and/or increase inpotassium, may thus be used as an indication of ischemia, in the heart,in the brain, and in other muscles and organs. In one example, a pHISE/sodium ISE electrode pair can be used to monitor the pH in or nearthe heart over time, and generate and alarm or indication when asignificant drop in pH has occurred. A measurement in pH near or withinthe heart can provide a good, undiluted, pH measurement. Some methodsand systems include both the initial drop in pH and the later rise inpotassium, in the generated indications.

One indication of ischemia can include storage of a marker or flag in animplanted medical device, for later reading by telemetry and transmittalto a physician. Another indication can include an immediate transmittalvia wired or wireless communication to a device located outside of thepatient's body, which may immediately alert the patient. This device cantransmit the indication to a central location, for example, overtelephone lines, for interpretation by a physician.

The pH and potassium levels may be tracked as a ratio or trackedeffectively as the pH and potassium, where the sodium level is measuredperiodically and/or inferred in the ratios to determine the pH andpotassium levels.

Ion-selective Electrode Membrane Preparation

Ion-selective membranes may be prepared using methods well known tothose skilled in the art. Recipes, materials, and methods are providedin the Fluka Chemica Selectophore catalog, available from Sigma-AldrichChemical. Some examples are given in PCT Publication No. WO 00/54039,herein incorporated by reference. Some information from WO 00/54039 isreproduced below.

Example 1 (Polymer Type I) Typical Recipe

-   n-butyl acrylate (BA)=2.5 g-   methylmethacrylate (MMA)=0.3 g-   2,2′-azobisisobutyronitrile (AIBN)=0.003 g-   benzene=3 ml-   ionophore (e. G. acrylolamidobenzo-15-crown-5 or 18-crown-6)=0.1 G-   measured polymer Tg=−29.4° C.    Procedure A: Polymerisation

The monomers, initiator and ionophore are mixed slowly with the solvent(benzene) and degassed with nitrogen for 10 minutes. The mixture isheated with stirring to 110° C. for 1 hour at which temperature itslowly thickens. The temperature is then lowered to about 70° C. whichtemperature is maintained for a further 6 hours to complete thereaction.

B. Isolation

About 5 ml of benzene is added to the cooled, sticky, clear, semi-solidand the mixture stirred vigorously. 1 ml of the clear sticky liquidproduced is added to 100 ML of petroleum ether (either 60° C. or 80° C.boiling fraction) with vigorous stirring until the solvent becomes clearand a stick, glue like solid is obtained. The solvent is poured off andthe solid washed with a further 20 ml of petroleum ether. The procedureis repeated with the remainder of the polymer solution until all of thepolymer has been precipitated.

C. Purification

All of the precipitated polymer is redissolved in about 10 ml of1,4-dioxane. 100 ml of distilled water is added to about 2 ml of polymersolution and a white sticky polymer appears. The procedure is repeatedfor the remainder of the polymer solution and all of the polymerportions are mixed and dried in a freeze drier for 2 hours. The whole ofthe polymer purification procedure as described above is repeated andthe pure polymer is then dried in a freeze drier under a vacuum of 10-2atmospheres for 24 hours. The final appearance of the polymer is usuallya colorless, clear, elastic and sticky solid although on occasions itmay appear slightly translucent. Yield 50 to 70%.

Possible Modifications

The following modifications may be made in the procedure described aboveas appropriate:

-   1. The solvent benzene (which is relatively toxic) may be replaced    by ethyl acetate;-   2. petroleum ether may be replaced by n-hexane;-   3. Both the isolation and the purification steps may be omitted in    which case the unpurified polymer mixture is diluted to the required    concentration for direct film deposition.    Fabrication of Membrane for Potassium Selective Electrode

A cocktail of polymer type I (about 0.2 g of the polymer) containing theappropriate lipophilic anion (for details see later) is prepared bydissolving the various components in about 1 ml of dichloromethane. Thecocktail is then poured onto a glass ring (QUICKFIT adapter) rested on aglass slide. The top of the ring is covered with a piece of filter paperand a heavy weight on top presses the paper against the ring. After 24hours of evaporation in a fume cupboard, the whole is immersed in waterfor 2 hours following which it is possible to remove the membrane fromthe glass slide and the membrane is ready for testing.

Polymer type I with immobilized potassium ionophore 18C6 Membranerecipe:

-   Non-plasticised polymer=0.15 g (BA=90 WT %, MMA=6.4 wt %)-   Immobilized 18-crown-6=1.9 wt %-   Potassium tetrakis (p-chlorophenyl borate)=1.7 wt % (60 mol %)-   measured polymer TG<−29.4° C.    Polymer type I with immobilized potassium ionophore 15C5 Membrane    recipe:-   Non-plasticised polymer=0.25 g (BA=88.6 wt %, MMA=7 wt %)-   Immobilized 15-crown-5=3.8 wt %-   Potassium tetrakis (p-chlorophenyl borate)=0.6 wt % (10 mol %)-   measured polymer Tg=−20.9° C.    Polymer Type II Typical Recipe:-   Inner layer: hydroxyethyl methacrylate=3 mg-   2,2-dimethoxy phenylacetophenone=1 wt %-   Ion-selective film: n-butyl acrylate=90 wt %-   hexanedioldiacrylate=0.1-0.3 wt %-   2,2-dimethoxy phenylacetophenone=1.6%.-   Measured polymer Tg=−44.1° C.-   Amount of ionophore varies depending on the type of ionophore.-   Amount of lipophilic additive varies depending on the ionophore.    Procedure A. Fabrication of Polyhema Layer

The surface of a Ag/AgCl button is wiped clean of dust with absoluteethanol. A solution of hydroxyethyl methacrylate which contains 1 wt %of the photoinitiator 2,2-dimethoxy phenylacetophenone (DMPP) isprepared and 3 mg of this solution is deposited on the button using aGilson pipet. Without delay the button is placed in a UV exposure unit(RS) and the unit is flashed with a stream of nitrogen before the lampis switched on for a duration of about 5 minutes with continued flashingwith nitrogen during which time polymerisation occurs. At the end of 5minutes, polymerisation is completed to form a hard, glassy, transparentpolymer film. The UV unit is switched off and thepolyhydroxymethacrylate (polyhema) film rinsed with a little absoluteethanol. The film is dried at room temperature for several minutesbefore a drop of salt solution (depending on the sensor type) is addedon the film and 2 hours are allowed for hydration.

B. Recipes for Ion-Selective Films

Depending on the type of ion sensing film, the monomer, ionophore andlipophilic additive used to prepare the ion-selective film can vary bothin type and concentration as follows:

-   Monomers: n-butyl acrylate-   methyl methacrylate-   n-heptyl acrylate-   cyanoethylacrylate-   Ionophores: potassium: acrylamidobenzol 5-crown-5-   (requires THF/benzene as solvent)-   valimomycin-   sodium: bis [(12-crown-4) methyl] dodecylmethylmalonate-   (Sodium ionophore VI)-   hydrogen: tridodecylamine-   (hydrogen ionophore I)-   calcium: N,N-dicyclohexyl-N′N′-dioctadecyl-3-oxapentamide-   (Calcium ionophore IV)-   lithium: 6,6-dibenzyl-14-crown-4-   (Lithium ionophore VI)-   Additives: potassium tetrakis (p-chlorophenyl) borate-   sodium tetrakis (trifluoromethylphenyl) borate.

Details of the recipes for various photocured ion-selective films aregiven below.

Fabrication of Ion-Selective Film

A cocktail which contains the monomer or monomers, cross-linkerhexanedioldiacrylate, photoinitiator DMPP, ionophore and lipophilicadditive is prepared and all of the components should form a homogeneoussolution. About 10 μl of this solution is drop coated on top of thehydrated polyhema layer with a Gilson pipet. The button is then exposedto UV radiation in an UV exposure unit for about 6 minutes (for mostcross-linked acrylate films) under continuous purging with nitrogen gas.The film formed is usually clear, soft and tacky but elastic. Thesurface of the film is rinsed with several drops of petroleum ether toremove unreacted substances.

The solid state electrode is ready for testing after encapsulation withARALDITE epoxy or fixed on to a static cell with an O-ring seal.

Possible Modifications Alternative initiators can be used includingbenzoyl peroxide and benzophenone.

Example 1 Potassium Ion-Selective Solid-State Electrode Based on PolymerType II with Immobilized 15C5 Evaluated under Constant Exposure of 0.1 MKCl Polyhema Layer:

-   hydroxyethyl methacrylate=6 mg-   2,2-dimethoxy phenylactophenone=1 wt %-   Duration of UV irradiation=10 minutes-   Conditioned in KCl for 2 hours-   Composition of photocured membrane (total weight 26 mg):-   n-butyl acrylate=83.9 wt %-   methyl methacrylate=12 wt %-   AAB 15C5=2.5 wt % (in 90 μl THF)-   potassium tertakis (p-chlorophenyl borate)=0.3 wt % (8.6 mole %)-   2,2-dimethoxy phenylacetophenone=0.9 wt %-   UV exposure time=40 minutes-   measured polymer Tg<−20.9° C.

Example 2 Potassium Ion-Selective Solid-State Electrode Based on PolymerType II with Entrapped Valinomycin Polyhema Layer

-   hydroxyethyl methacrylate=3 mg-   2,2-dimethoxy phenylactophenone=1 wt %-   Duration of UV irradiation=7 minutes-   Conditioned in KCl for 2 hours-   Composition of ion-selective membrane (total weight 15 mg):-   n-butyl acrylate=96 wt %-   hexanediol diacrylate=0.09 wt %-   valinomycin=2.1 wt %-   potassium tertakis (p-chlorophenyl borate)=0.59 wt % (60 mole %)-   2,2-dimethoxy phenylacetophenone=1 wt %-   UV exposure time=7 minutes-   measured polymer Tg=−42.6° C.

Example 3 Sodium Ion-Selective Solid-State Electrode Based on PolymerType II with Entrapped Sodium Ionophore IV Polyhema Layer

-   hydroxyethyl methacrylate=3 mg-   2,2-dimethoxy phenylactophenone=1 wt %-   Duration of UV irradiation=7 minutes-   Conditioned in NaCl 0.1 M for 2 hours-   Composition of ion-selective film (total weight 11 mg):-   n-butyl acrylate=94.7 wt %-   hexanediol diacrylate=0.11 WT %-   2,2-dimethoxy phenylacetophenone=0.72 wt %-   Na ionophore IV=4.3 WT %-   sodium tertakis (trifluorophenyl borate)=0.22 wt % (9 mole %)-   Duration of irradiation=8 minutes-   measured polymer TG=−42.6° C.

The above examples of ISE membrane recipes are only non-limitingexamples, with many other examples well known to those skilled in theart.

Thus, embodiments of the ION SENSOR FOR LONG TERM USE IN COMPLEX MEDIUMare disclosed. One skilled in the art will appreciate that the presentinvention can be practiced with embodiments other than those disclosed.The disclosed embodiments are presented for purposes of illustration andnot limitation, and the present invention is limited only by the claimsthat follow.

1. A method for measuring a potassium ion concentration in a human body,the method comprising: obtaining a differential electrical potentialbetween an implanted potassium ion-selective electrode (ISE) and animplanted sodium ISE; determining a sodium ion concentration near thesodium ISE; and determining a potassium ion concentration at least inpart as a function of the sodium ion concentration and the differentialelectrical potential.
 2. A method according to claim 1, wherein thesodium ion concentration determining includes measuring the sodium ionconcentration in human body fluids.
 3. A method according to claim 2,wherein the sodium ion concentration determining includes using afunction of at least the measured sodium ion concentration and a changein the differential electrical potential since the time of measuring thesodium ion concentration.
 4. A method according to claim 2, wherein thesodium ion concentration determining includes using a function of atleast the measured the sodium ion concentration and an expected changebased on therapy performed on the human body since the sodium measuring.5. A method according to claim 2, wherein the determining the potassiumion concentration includes application of a Nernst type equationutilizing the log of the ratio of the potassium and sodium ionactivities.
 6. A method according to claim 2, in which the body fluid isselected from the group: a volume of blood, a volume of urine, a volumeof spinal fluid.
 7. A method according to claim 2, in which the sodiumion concentration measuring utilizes the sodium ISE, in which the sodiumISE exhibits substantially sub-Nernstian response over a physiologicalrange of sodium ion concentration.
 8. A method for monitoringelectrolyte in a human body fluid, the method comprising: obtaining adifferential electrical potential from an electrode pair implanted inthe human body, the electrode pair including a potassium ion-selectiveelectrode (ISE) and a sodium ISE; and obtaining a ratio of potassium ionconcentration to sodium ion concentration at least in part as a functionof the differential electrical potential.
 9. A method according to claim8, further comprising determining a sodium ion concentration frommeasuring sodium ion concentration in the human body fluid, anddetermining a potassium ion concentration at least in part as a functionof the determined sodium ion concentration and the differentialelectrical potential.
 10. A method according to claim 8, in which theobtaining differential electrical potential is obtained from theelectrode pair being implanted in body fluid selected from the group: ablood sample, a urine sample, and a spinal fluid sample.
 11. A methodaccording to claim 8, further comprising repeating the obtaining stepsover time to monitor changes in the ratio over time.
 12. A methodaccording to claim 11, in which the body fluid is blood, furthercomprising generating an indication when the potassium to sodium ratiodrops below or exceeds a first threshold and a second threshold
 13. Amethod according to claim 11, in which the body fluid is blood, furthercomprising generating an indication when the potassium to sodium ratiorises above an upper threshold.
 14. A method according to claim 8, inwhich the obtaining a differential electrical potential utilizes thesodium ISE, in which the sodium ISE exhibits substantially sub-Nernstianresponse over a physiological range of sodium ion concentration.
 15. Animplantable electrode pair comprising: a potassium ion-selective workingelectrode and a sodium ion-selective reference electrode.
 16. Animplantable electrode pair according to claim 15, in which the potassiumion-selective electrode includes a first substrate, a firstmetal/metal-halide layer disposed on the first substrate, a first halidecontaining layer disposed over the first metal/metal-halide layer, afirst potassium ISE membrane disposed over the first halide containinglayer, wherein the sodium ion-selective electrode includes a secondsubstrate, a second metal/metal-halide layer disposed over the secondsubstrate, a second halide containing layer disposed over the secondmetal/metal-halide layer, a second sodium ISE membrane disposed over thesecond halide containing layer.
 17. An implantable electrode pairaccording to claim 16, in which the first and second halide containinglayers include a hydrogel and contain the same halide as the respectivemetal/metal halide layer.
 18. An implantable electrode pair according toclaim 17, in which the halide containing layers include KCl.
 19. Animplantable electrode pair according to claim 16, in which the first andsecond substrates are the same.
 20. An implantable electrode pairaccording to claim 16, in which the first and second metal/metal halidelayers have the same chemistry.
 21. An implantable electrode pairaccording to claim 16, in which the first ISE membrane has a potassiumion-selective ionophore and in which the second ISE membrane has asodium ion-selective ionophore.
 22. An implantable electrode pairaccording to claim 21, in which the first and second ISE membranesinclude a polymer, and in which the polymer is substantially the samefor both ISE membranes.
 23. An implantable electrode pair according toclaim 22, in which the first and second ISE membranes each include amobile plasticizer in the polymer.
 24. An implantable electrode pairaccording to claim 22, in which the first and second ISE membranes areeach self-plasticizing, and have essentially no mobile plasticizer inthe polymer.
 25. An implantable electrode pair according to claim 16, inwhich the potassium and sodium ISEs are each contained within a surfacearea of less than about 1 square centimeter.
 26. An implantableelectrode pair according to claim 16, in which the potassium and sodiumISEs each have no major dimension greater than about 1 centimeter. 27.An implantable electrode pair according to claim 16, in which the ISEmembranes are disposed directly on the halide containing layers.
 28. Animplantable electrode pair according to claim 16, in which the sodiumISE exhibits substantially sub-Nernstian response over a physiologicalrange of sodium ion concentration.
 29. An implantable electrode pairaccording to claim 16, in which the sodium ISE has a molar ratio ofionophore to lipophillic salt of between about 1 and 1.1.
 30. A methodfor monitoring a ratio of potassium ion to sodium ion concentration in ahuman being over time, the method comprising: measuring a differentialelectrode potential between a potassium ISE and a sodium ISE bothimplanted in the human being; and obtaining the ratio at least in partas a function of the measured differential electrode potential.
 31. Amethod according to claim 30, in which the measuring measures thedifferential electrode potential in blood.
 32. A method according toclaim 30, in which the measuring measures the differential electrodepotential in urine.
 33. A method according to claim 30, furthercomprising generating an indication when the ratio drops below a lowerlimit.
 34. A method according to claim 30, further comprising generatingan indication when the ratio rises above an upper limit.
 35. A methodaccording to claim 30, further comprising generating an indication whenthe ratio exhibits a rate of change greater than a rate of change limit.36. A method for controlling delivery of a substance into the humanbody, the method comprising: measuring a differential potential from anion electrode (ISE) pair implanted within the body, the ISE pairincluding a first ISE responsive to a first ion concentrationelectrically coupled to a second ISE responsive to a second ionconcentration; and varying delivery of the substance at least in part asa function of the measured differential electrical potential.
 37. Amethod according to claim 36, in which the first ion is potassium andthe second ion is sodium, in which the first ISE is a potassium ISE, inwhich the second ISE is a sodium ISE, in which the measuring includesmeasuring the differential electrical potential between the potassiumISE and the sodium ISE.
 38. A method according to claim 37, in which thevarying delivery includes varying infusion from an implanted deliverydevice operably coupled to the ISE pair.
 39. A method according to claim38, in which the substance includes a diuretic and in which the varyingdelivery includes varying delivery of the diuretic.
 40. A methodaccording to claim 38, in which the measuring includes measuring blood.41. A method according to claim 38, in which the measuring includesmeasuring urine.
 42. A method according to claim 38, in which themeasuring includes measuring both blood and urine.
 43. A methodaccording to claim 42, in which the measuring measures blood and urinenear the kidney.
 44. A method according to claim 38, in which theimplanted delivery device is disposed outside the body.
 45. A methodaccording to claim 38, in which the implanted delivery device isimplanted within the body.
 46. An implantable ion-selective electrodepair comprising: a first substrate having a first electrical conductorin ion communication with a first ion-selective layer, in which thefirst ion-selective layer includes a first material and a firstionophore, in which the first material is self-plasticizing; and asecond substrate having a second electrical conductor in ioncommunication with a second ion-selective layer, in which the secondion-selective layer includes a second material and a second ionophore,in which the second material is self-plasticizing.
 47. An implantableelectrode pair according to claim 46, in which the first and secondsubstrates are the same.
 48. An implantable electrode pair according toclaim 46, in which the first and second substrates are substantially thesame.
 49. An implantable electrode pair according to claim 46, in whichthe first and second materials are selected from the group consisting ofpolymer, glass, ceramic, and metal-oxide.
 50. An implantable electrodepair according to claim 46, in which the first and second materialsinclude a polymer.
 51. An implantable electrode pair according to claim46, in which the first and second ISE membranes consist essentially ofthe same composition, but have different ionophores as between eachother.
 52. An implantable electrode pair according to claim 46, in whichthe first and second ISE membranes have different ionophoreconcentrations as between each other.
 53. An implantable electrode pairaccording to claim 46, in which the first and second polymers have apolymer backbone and a plurality of pendant lipophilic plasticizinggroups.
 54. An implantable electrode pair according to claim 53, inwhich the polymer has a Tg of less than −10° C.
 55. An implantableelectrode pair according to claim 53, in which the lipophilicplasticizing groups include C3-C7 alkyl groups.
 56. An implantableelectrode pair according to claim 53, in which the lipophilicplasticizing groups include C3-C20 alkyl groups.
 57. An implantableelectrode pair according to claim 46, in which one of the ISEs is asodium ISE and exhibits substantially sub-Nernstian response over aphysiological range of sodium ion concentration.
 58. An implantableelectrode pair according to claim 46, in which one of the ISEs is asodium ISE, and in which the sodium ISE has a molar ratio of ionophoreto lipophillic salt of between about 1 and 1.1.
 59. A method fordetecting ischemia comprising: obtaining a differential electricalpotential between an implanted electrode pair in the human body, theelectrode pair including a first ISE and a sodium ISE; and generating anindication of ischemia at least in part as a function of a change in thedifferential electrical potential over time.
 60. A method according toclaim 59, in which the first ISE is a pH ISE, in which the change is adrop in pH over time.
 61. A method according to claim 59, in which thefirst ISE is a potassium ISE and in which the change is a rise in aratio of potassium to sodium over time.
 62. A method according to claim59, in which the obtaining includes the electrode pair being implantedin a heart in the human body.
 63. A method according to claim 59, inwhich the ischemia detected is a cardiac infarction.
 64. A methodaccording to claim 59, in which the ischemia detected takes place in abrain in the human body.
 65. A method according to claim 1, wherein atleast one of the implanted potassium ISE and the implanted sodium ISEare adapted to be disposed at least partially in a region of relativelyhigh fluid flow.
 66. A method according to claim 65, wherein said regionof relatively high fluid flow comprises a portion of: a Superior VenaCava vessel, an Inferior Vena Cava vessel, a right ventricular outflowtract.
 67. A method according to claim 8, wherein at least one of theimplanted potassium ISE and the implanted sodium ISE are adapted to bedisposed at least partially in a region of relatively high fluid flow.68. A method according to claim 67, wherein said region of relativelyhigh fluid flow comprises a portion of: a Superior Vena Cava vessel, anInferior Vena Cava vessel, a right ventricular outflow tract.
 69. Amethod according to claim 15, wherein at least one of the potassiumion-selective working electrode and the sodium ion-selective referenceelectrode is adapted to be disposed at least partially in a region ofrelatively high fluid flow.
 70. A method according to claim 69, whereinsaid region of relatively high fluid flow comprises a portion of: aSuperior Vena Cava vessel, an Inferior Vena Cava vessel, a rightventricular outflow tract.
 71. A method according to claim 30, whereinat least one of the potassium ISE and the sodium ISE are adapted to bedisposed at least partially in a region of relatively high fluid flow.72. A method according to claim 71, wherein said region of relativelyhigh fluid flow comprises a portion of: a Superior Vena Cava vessel, anInferior Vena Cava vessel, a right ventricular outflow tract.
 73. Amethod according to claim 59, wherein at least one of the first ISE andthe sodium ISE are adapted to be disposed at least partially in a regionof relatively high fluid flow.
 74. A method according to claim 73,wherein said region of relatively high fluid flow comprises a portionof: a Superior Vena Cava vessel, an Inferior Vena Cava vessel, a rightventricular outflow tract.